(1) Field of the Invention
The present invention relates to a Cathode-Ray Tube (CRT) device used, for example, in a computer and a television set, and in particular to a technique to improve the resolution.
(2) Description of the Related Art
A CRT device used in a computer monitor and a television set emits an electron beam from an electron gun onto a phosphor screen where the phosphor emits light to display an image. Resulting resolution is higher when a spot of the electron beam (hereinafter referred to as a xe2x80x9cspot diameterxe2x80x9d) causing the light emission of the phosphor screen is smaller.
FIG. 1 is a vertical sectional view schematically showing the construction of a typical electron gun. In FIG. 1, an electron gun 1 is constructed of a cathode 10, a control electrode 11, an accelerating electrode 12, a focusing electrode 13, and a final accelerating electrode 14. Electrons emitted from the cathode 10 are accelerated by the accelerating electrode 12 toward a phosphor screen 15. The focusing electrode 13 and the final accelerating electrode 14 together generate an electrostatic lens (hereinafter, referred to as a xe2x80x9cmain lensxe2x80x9d) 16 to bring the focus of the electron beam onto the phosphor screen 15.
Generally, the spot diameter varies in relation to the diameter of the electron beam within the main lens (hereinafter, referred to as the xe2x80x9cbeam diameterxe2x80x9d). Such a beam diameter that results in the smallest spot diameter is referred to as an optimum beam diameter. The optimum beam diameter differs depending on a luminance signal, that is a current flowing through the cathode (hereinafter, referred to as a xe2x80x9cbeam currentxe2x80x9d) Ia.
Here, it is known that if an electron gun is designed to be optimum in the beam diameter when the beam current Ia is large i.e. when the luminance is high, such an electron gun does not results in the optimum beam diameter when the beam current Ia is smaller. Conversely, however, if an electron gun is designed to be optimum in the beam diameter when the beam current Ia is smaller, the resulting beam diameter falls short of optimum when the beam current is larger.
To address the above problems, conventional electron guns are designed to be optimum in the beam diameter when the beam current is large in view of the fact that when the beam current Ia is larger, the beam diameter generally tends to be larger. With such a construction, however, the beam diameter still falls short of optimum when the beam current is small, which means that when the luminance of an image is low, the resolution becomes low. As a result, for example, a dim and fine pattern may not be displayed in detail.
Another conventional electron gun disclosed in Japanese Patent Unexamined Publication No. HEI 7(1995)-085812 is provided with an auxiliary electrode arranged between the accelerating electrode and the focusing electrode. The auxiliary electrode is applied with a voltage that has been adjusted according to a luminance signal so as to vary the strength (converging action) of pre-focusing lens. As a result, the beam diameter is controlled in a manner to minimize the spot diameter.
To meet this end, however, a high voltage ranging from hundreds to thousands volts needs to be applied to the auxiliary electrode. Consequently, in order to vary the strength of pre-focusing lens in a manner of allowing an image within which the luminance greatly and frequently differs to be displayed in high resolution, the electron gun is required to have an amplifier circuit capable of varying such a high voltage with high speed (several MHz to tens of MHz), which is not practical at all.
The present invention is made in view of the above problems, and with an object of providing a CRT device for displaying an image in high resolution irrespective of the luminance.
To achieve the above object, a CRT device of the present invention comprises: an electron gun including a cathode and a control electrode; and spot diameter control means for controlling a spot diameter by varying a difference between a cutoff voltage of the cathode and a voltage applied to the control electrode in accordance with a luminance signal. With this construction, the strength of cathode lens is varied by controlling the difference between the cutoff voltage of the cathode and the voltage of the control electrode in accordance with the luminance. As the strength of cathode lens is varied, the point of crossover shifts, which in the end controls the spot diameter, thereby improving the resolution of the CRT device.